Sounds like you already know what it is. It appears to be an 1898 rifle that's had the barrel cut down and the wood cut down. The hand guard has been discarded and the rear sight removed. The front sight appears to be a 1903 Springfield sight and the rear is a peep mounted to a fixture in the magazine cut off hole. The scope mount also has been drilled and tapped into the receiver so you can't see what model it is without removing that...which you've done to read the serial number. The addition of the pistol grip to the small of the butt was fairly common. As you know it's 30-40 caliber...the sling appears to be a cast off Russianicon... Shooter's value only on this one.